The present invention relates to a flow meter which is operable magnetically to detect a body which is movable in response to a change of flow rate of refrigerant gas being discharged in a variable displacement compressor.
Japanese Patent Application Publication No. 2002-332962 discloses a variable displacement compressor (hereinafter referred to merely as compressor) in which the inclination angle of a swash plate is varied by adjusting the opening degree of a control valve, thus the displacement of the compressor being changed. In the present specification, the displacement of the compressor refers to the flow rate or the volume of refrigerant gas discharged per one rotation of the compressor.
In a conventional compressor, although a flow rate changing command is merely sent in controlling and changing the displacement, actual displacement is not known. As the displacement is changed, and the power of the compressor is varied, actua1 displacement is estimated by a calculated value based on a flow rate command value.
Thus, the actual displacement is different from the calculated value until the displacement reaches a specified value after the flow rate changing command has been sent. Especially, when the compressor is started at start-up of a vehicle engine, the above difference is increased. Thus, it takes a longer time for the vehicle interior temperature to reach a desired level, and a greater load acts on the vehicle engine. Namely, appropriate control is hard to be performed under this situation.
If a flow rate of the refrigerant gas being discharged in the compressor is accurately detected, the actual displacement and the actual power of the compressor can be known, which is very useful. For the above purpose, an electric flow meter which is disclosed in Japanese Utility Model Application Publication No. 60-152926 (cf. FIG. 2) may be used for detecting the flow rate of the refrigerant gas.
The electric flow meter of Japanese Utility Model Application Publication No. 60-152926 has a valve seat formed in the horizontal partition wall of a box and having a valve hole. The space above the horizontal partition wall is divided into two spaces by a drive plate which is integral with a movable membrane. A valve body is connected to the drive plate and urged by a spring in the direction which causes the valve body to be inserted into the valve hole. The space between the horizontal partition wall and the drive plate is in fluid communication with a fluid passage through which high-pressure fluid flows. A part of low-pressure fluid which has passed through the valve hole is supplied to the space above the drive plate through a communication passage.
Thus, the valve body is movable vertically in the valve hole by the differential pressure between the high-pressure fluid in the spaces below the drive plate and low-pressure fluid in the space above the drive plate thereby to change the clearance between the valve seat and the valve body, thus forming a variable throttle. The displacement of the valve body by the above differential pressure is detected by a transducer which sends out an electric signal corresponding to the flow rate.
However, the electric flow meter of Japanese Utility Model Application Publication No. 60-152926 has the valve seat and the valve body cooperating to form a variable throttle, the movable membrane and the drive plate for supporting the valve body, and means for defining the spaces for the high-pressure and low-pressure fluids. The electric flow meter is made inevitably large in size. Thus, the electric flow meter is hard to be mounted on a compressor which is installed in a limited space of a vehicle.
In view of a device, the above downsizing problem in Japanese Utility Model Application Publication No. 60-152926 can be solved by providing a variable throttle mechanism formed by a combination of a flow passage for refrigerant gas and a movable body such as a spool or the like so that a differential pressure is created between the two points for moving the movable body.
In the above manner of sliding the movable body, however, the flow passage of the refrigerant gas has a gap formed between the sliding portion of the movable body and the inner surface of the flow passage. Thus, though depending on a setting, the fluid leaks easily through the gap of the sliding portion. Especially, the differential pressure cannot be large enough in a low flow rate region of the compressor immediately after the compressor has been started up. Thus, the differential pressure is not changed in a linear manner.
The diameter of the flow passage for the refrigerant gas may be reduced as much as possible for alleviating the influence of fluid leak thereby to increase the differential pressure between the two points and to change the differential pressure in a linear manner in the low flow rate region. However, since the flow of the fluid is substantially throttled in the low flow rate region, the differential pressure becomes beyond restraint or it is changed in a nonlinear manner in a high flow rate region of the compressor. Thus, when downsized, the conventional flow meter cannot accurately detect the flow rate.
When the diameter of the flow passage is reduced in the compressor for enhancing the throttling effect, resistance against the flow of fluid is increased, thus leading to reduction of cooling efficiency of the compressor. The reduction of cooling efficiency due to the increased fluid resistance cannot be overlooked particularly during the high load operation which requires a high flow rate for a large cooling capacity. On the other hand, when the diameter of the flow passage is increased in the compressor for ensuring sufficient cooling capacity in the high flow rate, the movable body must be made with a diameter that is larger than that of the flow passage for establishing a closed state of the flow passage and the flow meter is accordingly made large in size.
The present invention is directed to a flow meter which has a simple structure for use in a compressor, detects flow rate accurately in a wide range from a low flow rate region to a high flow rate region and prevents cooling efficiency from being lowered in the high flow rate region.